1. Field of the Invention
The present invention relates to initializing a digital equalizer in a physical layer transceiver configured for receiving network signals from a prescribed network medium, such as a 100Base-TX network medium.
2. Background Art
Local area networks use a network cable or other network media to link nodes (e.g., workstations, routers and switches) to the network. Each local area network architecture uses a media access control (MAC) enabling network interface device at each network node to share access to the medium.
Physical (PHY) layer devices are configured for translating digital packet data received from a MAC across a standardized interface, e.g., a Media Independent Interface (MII), into an analog signal for transmission on the network medium, and recovering digital packet data from the analog signals transmitted from a remote node via the network medium. An example is the 100Base-TX Ethernet (IEEE Standard 802.3) receiver, configured for receiving an encoded analog signal at a 100 Mb/s data rate. The 100Base-TX Ethernet receiver typically will include a slicer to recover symbols from digitized samples of the analog signal.
One problem associated with recovering packet data from a wired network medium involves the need to overcome intersymbol interference encountered during transmission of packet data at longer cable lengths. For example, data communications using 100Base-TX may be implemented using a cable having any length between 0 and 100 meters. Since the cable length is variable, an adaptive arrangement is needed to determine the amount of equalization necessary to overcome intersymbol interference.
Digital receivers typically perform equalization to overcome intersymbol interference using decision feedback equalizers having a combination of a feedforward filter and a feedback filter. The feedforward filter and feedback filter typically are finite impulse response (FIR) filters controlled by a set of FIR coefficients. The optimal coefficients for the feedforward filter and the feedback filter typically are determined using an adaptive algorithm, known as the Least Mean Squares (LMS) algorithm. In particular, the LMS algorithm operates to minimize Mean Square Error by adjusting the values of the FIR filter coefficients.
Many networking environments, however, such as the 100 BaseT Ethernet environment, do not provide a known training sequence to enable the LMS algorithm to select FIR filter coefficients based on prescribed symbol values, requiring the digital receiver to “blindly” (i.e., without a known training sequence) determine the FIR filter coefficients. Moreover, the intersymbol interference encountered during transmission on the network medium renders the symbol data output by the slicer too unreliable for successful completion of a decision directed algorithm, such as the LMS algorithm. Hence, the PHY receiver in some cases may not be able to rely on symbol data to select FIR coefficients that reliably overcome intersymbol interference.